The present invention relates to a combustion chamber of an internal combustion engine.
A combustion chamber structure of an internal combustion engine is known to improve a combustion rate of mixture in the internal combustion engine as shown in, for example, Japanese Utility Model Application Laid-Open No. SHO 60-102428.
The combustion chamber structure of the internal combustion engine is as follows. A circumferential edge portion of a top of a piston and a circumferential edge portion of an inner wall of a cylinder head are projected with each other so that, when the piston is located in the vicinity of the top dead center, the circumferential edge portion of the top of the piston and the circumferential edge portion of the inner wall of the cylinder head are close to each other to form a squish area for generating a so-called squish flow.
Then, in the combustion chamber, a spark plug is disposed in the inner wall surface of the cylinder head more on the exhaust valve side than the position where the squish flow collides, directed to the center of the combustion chamber from the squish area.
With such a structure of the combustion chamber of the internal combustion chamber, in a final stage of a compression stroke, the circumferential edge portion of the inner wall surface of the cylinder head and the circumferential edge portion of the top of the piston are close to each other so that the squish area is narrowed. Accordingly, air or mixture located in the squish area is directed to the central portion of the combustion chamber in the form of the squish flow.
The squish flows directed to the central portion of the combustion chamber collide with each other to generate a turbulence. By this turbulence, the combustion of the central portion of the combustion chamber is accelerated.
Next, when the internal combustion takes an expansion stroke from the compression stroke and the piston starts to be lowered, the squish area is expanded, a phenomenon which is referred to as a reverse squish in which the mixture is sucked in the expanded squish area is generated. When the mixture is sucked into the squish area by the reverse squish, a flame generated by the spark plug also reaches the vicinity of the squish area from the central portion of the combustion chamber by the abovedescribed reverse squish. Then, when the squish area is expanded in accordance with the further lowering movement of the piston, the above-described flame is sucked into the squish area to thereby burn the mixture within the squish area.
The above-described combustion chamber structure of the internal combustion engine is thus used for accelerating the combustion of the portion where the flame propagation is slow, thereby making uniform the flame propagation, i.e., for reducing the difference in the combustion completion timing at each circumferential portion.
By the way, an intake opening portion for introducing new air and mixture into the combustion chamber and an intake valve for opening/closing the intake opening portion are provided in the cylinder head. Thus, since a temperature of the vicinity of the intake opening portion is lowered by the introduction of the cold new air and mixture, it is necessary to accelerate the combustion in this portion.
However, since it is necessary to provide a clearance to some extent in order to avoid the contact between the intake valves and the piston top surface, in between the vicinity of the intake opening portion and the piston top surface, it is impossible to form the squish area of the suitable clearance and it is difficult to accelerate the combustion in the vicinity of the intake opening portion. As a result, there is a fear that the mixture residing in the vicinity of the intake opening portion is self-ignited before the flame generated by the ignition of the spark plug has reached the place, resulting in a knock of the combustion.
In view of the above-described defects, the present invention has been made, and therefore has an object of the present invention to provide a technology to prevent generation of a knock of combustion while making the flame propagation uniform by accelerating the combustion in the portion where the flame propagation is slow as in the vicinity of the intake opening portion.
In order to solve the above-noted defects, the following means is adopted according to the present invention.
A combustion chamber structure for an internal combustion engine according to the present invention, a longitudinal sectional shape of a top wall surface of a combustion chamber is in the form of a substantially triangular shape in longitudinal section in one direction passing through a center of the combustion chamber defined and surrounded by a cylinder head, a cylinder and a piston, with a spark plug being disposed at an apex portion of the triangular shape and an intake opening portion and an exhaust opening portion on the top surface of the combustion chamber, and is characterized in that:
a projection is provided on a circumferential edge portion of a top surface of the piston, with a surface, facing the top wall surface of the combustion chamber, of the projection being substantially in parallel with the top wall surface of the combustion chamber; and
a cutaway portion is formed in the vicinity of at least a portion, facing the intake opening portion, of the projection of the top surface of the piston.
In this combustion chamber structure of the internal combustion engine, when the internal combustion chamber takes a shift in latter half of the compression stroke, the piston is raised up to the vicinity of the top dead center, the projection of the top surface of the piston and the top wall surface of the combustion chamber are close to each other to form a squish area. Then, when the piston is further raised, the squish area is narrowed so that the air, the mixture and the like that are residual in the squish area are advanced toward the central portion of the combustion chamber in the form of squish flows.
In this case, since the surface, facing the top wall surface of the combustion chamber, of the projection of the top surface of the piston is formed substantially in parallel with the top wall surface of the combustion chamber, the squish flows are advanced substantially in parallel with the top wall surface of the combustion chamber to flow toward the apex portion of the combustion chamber, i.e., the vicinity of the spark plug. Then, the squish flows that have been advanced close to the spark plug are brought into collision with each other to form the turbulence in the vicinity of the spark plug so that the combustion in the vicinity of the spark plug is accelerated.
Also, the air and mixture located in the squish area in the vicinity of the cutaway portion is introduced into the cutaway portion and collide with each other to generate the turbulence.
Then, when the internal combustion engine takes a shift from the compression stroke to the expansion stroke, and the piston starts to be lowered, the squish area is expanded so that the reverse squish flows are generated toward the squish area from the central portion of the combustion chamber. The reverse squish flows are advanced substantially in parallel with the top wall surface of the combustion chamber.
According to the present invention, since the surface, facing the top wall surface of the combustion chamber, of the projection of the top surface of the piston is formed substantially in parallel with the top wall surface of the combustion chamber, the reverse squish flows are strongly sucked into the squish area.
The reverse squish flows that have been sucked in the squish area reach the cylinder wall surface within the squish area and change their directions at the reached cylinder wall surface to advance in the circumferential direction along the wall surface of the cylinder.
The reverse squish flows that have changed the directions in the squish area around the cutaway portion are introduced into the cutaway portions and collided with each other to generate the turbulence. The turbulence becomes strong together with the turbulence generated in the previous compression stroke to accelerate the mixture residing around the cutaway portion, i.e., the mixture residing in the vicinity of the intake opening portion to thereby make uniform the flame propagation in the combustion chamber.
Accordingly, the mixture located in the vicinity of the intake opening portion where it is said that the flame propagation is slowest is burnt before the self-ignition to thereby suppress the generation of knocks.
Incidentally, the cutaway portion according to the present invention, formed in the vicinity of the portion facing the intake opening portion, may serve also as a valve recess in the projection of the top surface of the piston for avoiding any contact between the intake valve for opening/closing the intake opening portion and the piston.
Subsequently, in the case where the internal combustion engine is provided with a swirl flow generating means for generating swirl flows within the combustion chamber, the cutaway portion may be constructed by a groove formed in a direction in which the swirl flows generated by the swirl flow generating means are received so that the swirl flows are introduced into the cutaway portion, or the cutaway portion may be constructed by a groove formed in a direction in which the swirl flows generated by the swirl flow generating means are not received so that the swirl flows are not introduced into the cutaway portion.
Also, in the case where the internal combustion engine is provided with a tumble flow generating means for generating vertical eddy flows (tumble flows) within the combustion chamber, the cutaway portion may be constructed by a groove formed in a direction in which the tumble flows generated by the tumble flow generating means are received so that the tumble flows are introduced into the cutaway portion, or the cutaway portion may be constructed by a groove formed in a direction in which the tumble flows generated by the tumble flow generating means are not received so that the tumble flows are not introduced into the cutaway portion.